As a model F lover, let me respectfully suggest that you NOT bother to upgrade. You would only wind up obsessed with it and struggling to make a Model F fit your needs.
If you tried a model F, you would like it, but all Model Fs have their problems and quirks.
No.1 is the giant, the F122. These have a tendency not to come to you in good condition. I ain't fohat, I'm not opening them to fix hammers that got moved during shipping. If your F122 comes to you with that problem, have fun fixing your F. You will scream Fxxx 122 times. Actually I haven't tried to fix my F122, although I have screamed Fxxx at least 20x with my F XTs since I tried putting them together about 20x. F122 is not something that an ordinary geek has the strength to handle. Fohat is not a weak ordinary geek, so he can handle it.
No.2 is the F AT. Nice layout, but spacebar needs both thumbs to press and not to mention there is that gigantic ancient return key made for people with F AT little fingers and lack of modern ctrl-alt-windows keys on the bottom row.
No.3 is the F XT. Very entertaining layout and a similarly entertaining spacebar. Are you buying a keyboard to play with, or are you expecting to do serious work on your keyboard?
No. 4 is the F AT with a serial connector. You better know programming if you want to make it work with a soarer. Somebody here has managed but I can't remember his name.
No. 5 is the F Unsaver that is a Model C. Saves you no space, weird layout, you need to do a heck lot of work to make it work. I think somebody here has one that works but again I don't remember his name. It's not Dorkvader.
Dorkvader replied to your thread. I suggest you

his ass and make good friends with him. He's part of a team of heroes trying to bring about a Model F revival. Save your money and buy an F after Dorkvader and friends have managed to revive a non-infuriating user friendly Model F. Don't be an idiot like me and spend money and time struggling with the Fs and wind up with opened up Fs that don't work.
That said, this is typed on a model F XT. I found it impossible to get back to a Model M, even though I still make lots of mistakes on the spaces due to the stiff spacebar, and my " and / and returns are often screwed up. Yeah, don't be like me and start and get addicted.
Well, I am currently using my IBM Model M, and I love it! But, I would just like to know, for future reference. If there is a keyboard with buckling springs that feels better than the Model M. I heard the Model F felt even better so I have been watching that. Give me your ideas and opinions!